PURPOSE: To realize fail safe in the titled device, by using a window comparator for oscillating in the vicinity of an operating point, as a level detecting circuit, in a level detecting device provided with a drift compensating device.

CONSTITUTION: In a device for detecting a level of an input signal of a terminal 11 by a level detecting circuit 12, level detectors 20, 21, timers 22, 23 and a drift compensating device are provided, and a drift of a detecting side and a drift of a non-detecting side are corrected. The level detecting circuit 12 is a window comparator which has a window prescribed by the upper and lower threshold levels, and oscillates and provides an output only when the level of the input signal is within a range of the window, no detecting output is generated even if compensation is delayed for a long time due to a fault of the drift compensating device, and the device becomes fail safe.